1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus having the same, and is suitable for a digital camera, a video camera, a silver salt film camera, a TV camera, or the like.
2. Description of the Related Art
An image pickup apparatus such as a video camera or a digital still camera using a solid-state image pickup element needs to be small as a whole and to have a high performance. Further, an image pickup optical system used for the image pickup apparatus needs to be a small-sized zoom lens having a high optical performance in which a zoom ratio is around three times.
A negative-lead type two group zoom lens which is constituted by a first lens group having a negative refractive power and a second lens group having a positive refractive power in order from an object side to an image side and performs a zooming operation by changing an interval of each lens group is known.
For example, a zoom lens that is a negative-lead type two group zoom lens having a zoom ratio of around three times and that the first lens having the negative refractive power is constituted by two lenses and the second lens group having the positive refractive power is constituted by three lenses is known (Japanese Patent Laid-open No. 2007-293368).
Further, a zoom lens where the first lens group having the negative refractive power is constituted by two lenses and the second lens group having the positive refractive power is constituted by two lenses is known (Japanese Patent Laid-open No. 2005-148435).
Since the negative-lead type two group zoom lens has an asymmetric lens configuration, various kinds of aberrations caused by the asymmetry of the lens configuration such as a field curvature and a coma aberration are easily generated. In particular, when the size of a whole lens system is reduced and a zoom ratio is greater, the asymmetry is greater and it is difficult to correct these various kinds of aberrations.
Because a zoom lens disclosed in Japanese Patent Laid-open No. 2007-293368 has a meniscus shape where a concave surface of a negative lens of a first lens group faces an image side and a lens effective diameter is easily increased, a whole of the first lens group is thickened and the size of a whole lens tends to be larger.
A zoom lens disclosed in Japanese Patent Laid-open No. 2005-148435 uses an aspherical surface whose displacement from a spherical surface is large as a negative lens constituting a second lens group to correct the various kinds of aberrations. The negative lens has a lens shape where aberrations are exchanged by a lens surface at the object side and a lens surface at the image side to cancel the aberrations each other to make a whole balance with regard to components of the coma aberrations or the field curvature. Therefore, if a lens surface of the negative lens is decentered, the balance for cancelling the aberration is lost and the optical performance tends to be greatly deteriorated.
Commonly, in the negative-lead type two group zoom lens, in order to reduce a whole size and to ensure a predetermined zoom ratio to obtain a high optical performance, it is important that the lens configuration of the first lens group having a negative refractive power and the second lens group having a positive refractive power is appropriately formed.
In particular, in order to correct the field curvature and the coma aberration in a whole zoom range and to reduce the size of a whole optical system, it is important that a shape of the first lens surface arranged closest to the object side of the first lens group and a shape of a negative lens of the second lens group are appropriately formed.